Inkjet printers are commonplace in the computer field. These printers are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. Inkjet printers produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes a printing medium, such as paper.
An inkjet printer produces a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes "dot locations", "dot positions", or "pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet printers print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more print cartridges each having a printhead with ink ejecting nozzles. The carriage traverses over the surface of the print medium. An ink supply, such as an ink reservoir, supplies ink to the nozzles. The nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller. The timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
In general, the small drops of ink are ejected from the nozzles through orifices by rapidly heating a small volume of ink located in vaporization chambers with small electric heaters, such as small thin film resistors. The small thin film resistors are usually located adjacent the vaporization chambers. Heating the ink causes the ink to vaporize and be ejected from the orifices.
Specifically, for one dot of ink, an electrical current from an external power supply is passed through a selected thin film resistor of a selected vaporization chamber. The resistor is then heated for superheating a thin layer of ink located within the selected vaporization chamber, causing explosive vaporization, and, consequently, a droplet of ink is ejected through an associated orifice of the printhead.
However, there are several concerns that exist for controlling inkjet quality. First, as each droplet of ink is ejected from the printhead, some of the heat used to vaporize the ink driving the droplet is retained within the printhead. This heat can gradually build, eventually altering ejection performance. Namely, printhead overheating can occur when numerous nozzles are being fired during high density printing or when the firing frequency is increased during high speed printing. If the printhead reaches an overheating threshold temperature, print quality will be degraded and the inkjet printing process will be compromised. In fact, an increase in printhead temperature over the threshold temperature is directly related to an increase in dot or pixel size, which creates uneven printed dots or pixels, and thus, poor print quality. In addition, in extreme cases, an overheated printhead can cause the nozzles to misfire or cease from firing completely, thereby severely impairing further operation. Therefore, heat regulation is an important factor for controlling print capacity, output quality, and speed of most inkjet printers.
Next, since the printhead nozzles have relatively small flow areas, the nozzles are susceptible to clogging from contaminant particles. In addition, during high capacity or high speed printing, the sensitivity to fine particles is increased. One source of particulate contamination is from printhead manufacturing and assembly. Also, the ink and ink supply can contain particulate contamination. Although filters have been used, many either do not filter enough or micro fine particulate contamination, or are too restrictive, thereby hindering the ink flow, which can compromise print quality and print speed. As such, higher print quality can be achieved if the nozzles are free from particulate contamination and ink flow is not unduly restricted by a filtration system.
Therefore, what is needed is a thermally efficient filtration system for a printhead portion of an inkjet printer that can regulate printhead temperatures and filter particulate contamination without unduly restricting ink flow. What is, also needed is a thermally efficient filtration system that operates at very high throughput rates.